Many electronic devices are arranged to be connected to another device, such as a personal computer, laptop computer, or a printer, for data communication. Some examples for such devices are handheld computers, digital cameras, media players, or mobile phones. The connection to a PC or some other device is frequently effected via a serial data communication bus.
A serial data bus interface usually includes at least one data line, a ground line and a power line. Additional pins and lines may be present. In most cases a device is allowed to draw power from the power line on the interface. The purpose of this power supply is that devices with limited power consumption would not need to have an additional power supply for operation with the host device. For the example case of the Universal Serial Bus (USB) standard, specifications define how much power may be consumed by a device and under what conditions power may be drawn. Usually a specific connection procedure compliant with a respective standard is required before the serial bus can be used for data communication or for supplying power.
As most devices as mentioned above use a battery or rechargeable battery as a mobile power source, power consumption and recharging of batteries are important design considerations. The power supplied on a serial data bus might be used to recharge a battery utilizing the same interface and connection that are used for data communication through the bus.
A variety of electronic devices include some security or authentication lock, such as a personal identification code or password a user has to enter before he can access and utilize the device. This is a common feature for e.g. mobile phones, where a security lock is usually implemented via a numerical PIN code that has to be entered when the phone is powered on. If a user does not enter a valid PIN code, the various functions of the phone cannot be used.
If a user only needs limited access to a device, it is not always necessary to provide complete functionality for all units and elements of the device. For example, charging of an internal device battery or transferring data from the device memory via the serial bus interface does not require complete access to all functions and processes the device is capable of Besides the fact that handling would be easier and faster (for both the user and an external device connected for charging or data transfer) when no elaborate activation procedures have to be followed, also power consumption may be considerably lower when only part of a device is activated. However, to access and use a serial data bus interface such as USB, the device has to be powered on as described to be able to authenticate or initializes according to any predefined standard procedures.